1. Field of the Invention
This invention relates to a method for removing germanium oxides, more particularly to a method for removing germanium suboxide (GeOx, x<2) using a supercritical fluid composition.
2. Description of the Related Art
With the trends toward miniaturization and multi-functionality in electronic products, in the manufacturing techniques of integrated circuits (ICs), sizes of transistors have to be minimized to meet the demands for high performance and density. However, performance of a conventional MOSFET containing polysilicon/silicon dioxide/silicon channel is unlikely to be improved by virtue of mere miniaturization due to the limitations of carrier mobility of silicon substrate and capacitance of a gate dielectric layer. Thus, much research has been devoted to the development of a semiconductor substrate with high carrier mobility and of a high capacitance material for the gate dielectric layer.
In the research of promoting the carrier mobility of a semiconductor substrate, since the mobilities of electrons and electric holes of germanium (Ge) are respectively double and quadruple of those of silicon (Si) and since the manufacturing process for Ge is similar to that for silicon (Si), Ge-MOSFET is regarded as the semiconductor element having most potential in the future. Besides, in recent years, many kinds of dielectric materials that can be used in a gate dielectric layer of Ge-MOSFET to promote capacitance coupling ability have been developed, such as germanium dioxide (GeO2), silicon dioxide (SiO2), hafnium dioxide (HfO2), zirconium dioxide (ZrO2), and lanthanum oxide (LaO).
However, the aforesaid Ge-MOSFET is currently not widely used. One major reason is that germanium dioxide has inferior thermal stability compared to that of silicon dioxide (SiO2). Accordingly, when depositing and forming on a Ge substrate a dielectric layer composed of a material having a high dielectric constant in a subsequent manufacturing process under a relatively high temperature (about 400° C.) or when conducting a subsequent thermal process (>500° C.), GeO2 formed between the Ge substrate and the dielectric layer is likely to react with Ge of the Ge substrate to form germanium suboxide (GeOx, x<2) having a relatively low dielectric constant, thereby resulting in increased gate leakage current and reduced capacitance coupling. Therefore, how to reduce and remove germanium suboxide to alleviate the gate leakage current is the subject of endeavor in the Ge-MOSFET.
Two procedures for removing GeOx are currently available:
(1) Pre-treatment of a Ge-substrate before depositing a dielectric layer so as to remove native oxides on the Ge-substrate. For example, a method for preparing a surface of a semiconductor substrate is disclosed in U.S. Pat. No. 7,132,372B2, in which native oxides on the Ge-substrate are removed using oxygen plasma and wet etching. U.S. Pat. No. 7,238,291B2 disclosed a method for removing GeOx using an etching solution containing hydrogen bromide (HBr) and hydrogen iodide (HI). Besides, B. Xie et al. disclosed a method for removing native oxides from SiGe using a mixture of HF and water delivered by supercritical CO2 (scCO2) (B. Xie, G Montano-Miranda, C. C. Finstad, and A. J. Muscat, “Native oxide removal from SiGe using mixture of HF and water delivered by aqueous, gas, and supercritical CO2 process” Mater. Sci. Semicond. Process, vol. 8, pp. 231-237, 2005). In this literature, native oxide is removed by virtue of the etching property of HF. A mixture containing only water and scCO2, i.e., without HF, is unable to remove GeO2. Moreover, although the conventional method disclosed in the prior art can remove the native oxides on the Ge-substrate, the GeOx formed in a subsequent thermal process, e.g., between the Ge substrate and a dielectric layer, is unable to be removed using the mixture of HF and water since the etchant, HF, is unlikely to penetrate into an interface between the substrate and the dielectric layer.
(2) Removal of GeOx after depositing a dielectric layer. X. Zou et al. proposed use of a wet-N2 (water vapor/N2) annealing process under a relatively high temperature (500° C.) to remove GeOx formed between a Ge-substrate and a gate dielectric layer (X. Zou, J. P. Xu, C. X. Li and P. T. Lai, “Suppressed growth of unstable low-k GeOx interlayer in Ge metal-oxide-semiconductor capacitor with high-k gate dielectric by annealing in water vapor”, Appl. Phys. Lett., vol. 90, p. 163502, 2007). S. Rangan et al. proposed use of aluminum as a gate electrode that is deposited on a dielectric layer to draw the oxygen atom from GeOx to form aluminum oxide, thereby removing GeOx formed between a Ge-substrate and the dielectric layer (S. Rangan, E. Bersch, R. A. Bartynski, E. Garfunkel, and E. Vescovo, “GeOx interface layer reduction upon Al-gate deposition on a HfO2/GeOx/Ge(001) stake”, Appl. Phys. Lett., vol. 92, p. 172906, 2008). However, the former method for removing GeOx using water vapor/N2 at 500° C. requires a relatively high thermal budget, and the latter method for removing GeOx using aluminum has a disadvantage of lower capacitance coupling ability due to the formation of aluminum oxide having a relatively low dielectric constant.
Therefore, an effective method for removing GeOx to solve the problem of gate leakage current is still required in the Ge-MOSFET field.